The commercial production of chemical products almost always requires chemical analysis of various process streams to control the process and to ensure product quality. The chemical analysis can be off-line or on-line. An off-line analysis is accomplished by taking a sample of the process stream and then subjecting it to a laboratory analysis such as a titration. An on-line analysis is usually accomplished by conducting a portion of the process stream directly to a chemical process analyzer such as an on-line gas chromatograph. Some on-line chemical analyzers do not even require sampling the process stream such as by placing a pH electrode directly in a process stream. On-line chemical process analyzers play an important role in modern chemical production because they can reduce processing time, can increase product quality and can reduce costs.
On-line infrared analyzers are known and include analyzers based on Fourier Transform Infrared Spectroscopy (FTIR), see, for example, Chapter 10 of K. Clevett, Process Analyzer Technology (1986). U.S. Pat. No. 4,370,201 to Lowenhaupt discloses an on-line FTIR process for maintaining coal proportions in a coal blend. U.S. Pat. No. 4,743,339 to Faix et al. discloses an off-line FTIR process for controlling the digestion of wood pulp.
Polyolefins are often manufactured using a Ziegler type catalyst. An olefin, such as ethylene, is dissolved in a solvent and heated in a reactor under pressure in the presence of the catalyst to form the polyolefin. The polyolefin is then recovered and any unreacted olefin is recycled along with the solvent. Polyolefins have a large number of end uses such as in plastic bags and electrical cable insulation. Polyolefin manufacturers offer hundreds of different grades or types of their product to better meet the needs of these different uses. Often the difference between grades or types of a polyolefin product is a difference in the density of the product.
The density of a polyolefin manufactured using a Ziegler type catalyst can be controlled by dissolving a co-monomer, such as 1-butene, 4-methyl-1-pentene 1-hexene or 1-octene, in the solvent along with the principal monomer, such as ethylene. 16 Kirk-Othmer Encyclopedia of Chemical Technology, 385, at 388, also see 402, 421, 433, 453 and 470 (1981).
One means of controlling the process is to install flow meters/flow controllers on the various feed streams to the reactor. Product samples are taken and analyzed. When the desired product density is achieved, the flow meter outputs are noted and then used to automatically control the flow controllers. If the amount and composition of monomers being recycled is constant, then this type of process control can work well. However, the amount and composition of monomers being recycled is not constant because, for example, the activity of the catalyst changes. The output from the flow meters can also drift. The result of these situations can be a change in product density. When such a change is noted, then the flow controllers are adjusted to compensate so that the product density is brought back to the desired value. Polyolefin users have come to expect consistent product quality. If the density of the product changes enough before it is corrected, then the product may not meet specifications and may have to be sold as a lower grade for a lower price. Therefore, it would be an advance in this art if a process control method could be developed that resulted in less variation of product density during the production of a given type of product.
When it is desired to change the density of the product, the flow controllers are set to new values according to previous experience and samples of the product are taken and analyzed. The flow controllers are then fine tuned and when the desired new density is achieved, the flow meter outputs are noted and then used to automatically control the flow controllers. During the time that this change to the new density is being made, the product is likely to be of a lower grade which sells for a lower price. It would be an advance in this art if a process control method could be developed that resulted in a minimum of delay in achieving the new product density.